Modeling Hg(II) reduction through condensed phase photochemistry - - PowerPoint PPT Presentation

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

Modeling Hg(II) reduction through condensed phase photochemistry with dicarboxylic acids

Jesse Bash1, Annmarie G. Carlton2, William Hutzell1, Russell Bullock1

1. U.S. EPA/NERL 2. Rutgers University Department of Environmental Science

2011 CMAS Conference, October 25th Chapel Hill, NC

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

Outline

• Overview of aqueous phase reduction and motivation
• Model simulations

– Condensed phase reduction schemes

• None, HO2

., C2-C4 dicarboxylic acids (DCA)

– July-August and January-February simulations

• Evaluation against MDN observations

– Domain wide statistics – Regional differences

• Conclusions

2

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

Atmospheric Hg

• Mercury deposition is the primary source of mercury

contamination in ecosystems

– Reactive gas phase mercury, Hg(II), deposits readily – Gaseous elemental mercury, Hg(0), not as apt to deposit through wet and dry pathways – Atmospheric Hg is primarily (~99%) present as Hg(0)

• In soils or the water column Hg can be transformed

into organic Hg compounds

– Potent neurotoxins

• Hg(II) is reduced in cloud droplet to form Hg(0)

– Though mechanisms are not well-understood

3 Hg(II) Hg(II) Hg(0) Hg(0) Hg(II) is reduced to Hg0 during aqueous processing, but there is debate about the mechanism(s).

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

Motivation

• Most atmospheric models parameterize condensed

phase Hg(II) by HO2

. or scaled to OH

– Reduction by HO2

. has been shown to be improbable under

environmental conditions – Scaled rates do not represent real atmospheric chemical processes

• Recent laboratory experiments (Si and Ariya, 2008; Bartels-

Rausch et al 2011) and observations (Wang and Hintelmann, 2009)

indicate photoinduced reduction of Hg(II) by DCA

– Second order photoreduction of Hg(II) observed with oxalic, malonic and succinic acids

• A reduction mechanism proposed by Si and Ariya, 2008

is investigated in a regional scale modeling study

4

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

Model Simulations

• Changes to aqueous phase Hg chemistry

– CMAQ estimates cloud secondary organic aerosols (SOA)

• Oxalic acid is the dominant product of glyoxal oxidation and

dominates over other DCAs

– Same model run but the cloud SOA (primarily oxalic acid)

reduction pathway instead of HO2

.

• 2002 January-February and July-August model runs

– No condensed phase, HO2

. and DCA reduction mechanism

cases simulated – Evaluated against mercury deposition network (MDN) wet deposition observations

5

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

6

Domain Wide Evaluation

• Reduction scheme is

necessary to capture

• bservations
• DCA mechanism

reduced bias and error in wet deposition estimates in July- August simulations

• DCA mechanism

increased bias and error in wet deposition estimates in January- February simulations

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

7

Domain Wide Evaluation

r MB ME NMB NME MM5 Precip. Jan.

• Feb.

0.815 4.2 mm mon-1 16.9 mm mon-1 9.0% 37.5% Jul.

• Aug.

0.729 33.7 mm mon-1 59.3 mm mon-1 74.4% 132.3% No Reduction Jan.

• Feb.

0.726 0.423 μg m-2 mon-1 0.471 μg m-2 mon-1 111% 123% Jul.

• Aug.

0.787 1.233 μg m-2 mon-1 1.234 μg m-2 mon-1 117% 117% HO2 Reduction Jan.

• Feb.

0.718

• 0.014 μg m-2 mon-1

0.149 μg m-2 mon-1

• 4%

39% Jul.

• Aug.

0.655

• 0.428 μg m-2 mon-1

0.508 μg m-2 mon-1

• 41%

48% DCA Reduction Jan.

• Feb.

0.714 0.139 μg m-2 mon-1 0.231 μg m-2 mon-1 36% 60% Jul.

• Aug.

0.738

• 0.184 μg m-2 mon-1

0.381 μg m-2 mon-1

• 17%

36%

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

8

Jan.-Feb. Hg Wet Deposition

• Less variability between model simulations than July-August simulations
• DCA over predicted wet deposition in Gulf States
• Jan.-Feb. wet deposition more sensitive to boundary condition Hg(II)

concentrations

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

9

Jan.-Feb. Precipitation

• Precipitation is over estimated in the North East

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

10

July-August Hg Wet Deposition

• DCA mechanism reduces under prediction in Southeast and Midwest
• Increases wet deposition in the west and over the oceans where
• bservations are sparse
• No reduction scheme over predicts wet deposition domain wide

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

11

July-August Precipitation

• Precipitation is over estimated in the south and around Indiana
• Bias in TX and IA correspond to model over estimates in the DCA and no

reduction cases

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

Conclusions

• Condensed phase reduction of Hg(II) by DCA has been

parameterized in CMAQ – More probable than HO2

. and more physically descriptive than scaled

reduction mechanisms

• Improved July-August total Hg wet deposition performance

when compared to MDN observations – When Hg total deposition peaks in most locations

• Some degredation in January and February wet deposition

performance – Absolute increase in Jan. & Feb. bias is less than the improvements in

July & Aug.

– May be related to model boundary conditions

• Available in CMAQ 5.0 Multipollutant

12

Office of Research and Development Atmospheric Modeling Division, National Exposure Research Laboratory

Questions?

13